A tensioning device, such as a hydraulic tensioner, is used to impart and maintain a certain degree of tension in the chain or belt that drives, for example, an engine camshaft. A hydraulic tensioner typically comprises a housing having a bore, and a piston having a rear end which is inserted into the bore of the housing. The piston is biased in a protruding or outward direction by a spring. A fluid chamber is formed by the inner wall surface of the bore and the rear end face of the piston. A fluid pressure source provides pressurized fluid through a passage in the housing to the fluid chamber.
When air, a compressible fluid, is present in the fluid chamber, it can mix into the oil that is present in the fluid chamber. As the tension of the chain or belt increases, the air in the fluid chamber compresses, which allows compression of the normally incompressible oil, and the piston retracts. As a result, the chain or belt slackens, and proper functioning of the tensioner will not be achieved. It is therefore desirable to provide for venting of air from the fluid chamber.
Various hydraulic tensioners have been proposed to provide for venting of air from the fluid chamber to the outside of the housing. For example, a hydraulic tensioner using a sintered alloy filter for venting air is disclosed in Patent Showa 46-28575. Another hydraulic tensioner using a helical channel on its outer periphery for venting air is disclosed in Utility Model Patent Showa 60-30507. However, because these hydraulic tensioners require a separate air venting member such as a sintered filter or an air-extraction body, additional parts and complex construction are required.
The present invention is directed to the above-described problems in conventional hydraulic tensioners, and offers a hydraulic tensioner which can further reduce the number of parts and further simplify construction.